Multipole synchronous motor



Sept. 8, 1942. E. MICHELSEN 2,

MULTIPOLE SYNCHRQNOUS MOTOR Filed May 1, 1941 WITNESSES? 2 INVENTOR 2 fberfiard/y/bfifiserz ATTORNEY Patented Sept. 8, 1942 MULTIPOLE SYNCHBONOUS MOTOR Eberhard Michelson, Frohnan, Mark, Germany, assignor to Westinghouse Electric 8: Manufacturing .Company, East Pittsburgh, Pa., a corporation of Pennsylvania Appucation May 1, 1941, Serial No. 391,245 In Germany December 30, 1939 6 Claims.

The present invention relates to synchronous motors, and, more particularly. in multi-pole synchronous motors of relatively small size.

A principal object of the invention is to provide an improved low-speed, self-starting synchronous motor of the type which utilizes a permanent magnet to supply the necessary magnetization of the rotor. In motors of this type, as previously built, the magnetic path for the flux of the permanent magnet has been relatively long, usually extending from the magnet acrossan air gap to the rotor, and then across another air gap to the stator, and through the frame of the motor back to the magnet. Because of this long path, the magnetic losses are relatively high, and the reluctance of the magnetic circuit is also high, so that a relatively large magnet is required to obtain the desired total flux. With this arrangement, the magnetic flux from the perma-' nent magnet also links the stator windings, which are excited with alternating current, which is undesirable because of the damping effect on the stator flux.

A more specific object of the invention, there- I fore, is to provide a multi-pole, self-starting synchronous motor in which the magnetic path for the permanent magnet flux is relatively short,

and in which this flux does not link the stator windings. A

A further object of the invention is to provide a multi-pole synchronous motor of the type de-.

scribed above, in which the rotor member is divided into two axially separated sections which are magnetically insulated from each other, and

through which the flux of the permanent magnet passes in opposite directions, so that the flux re- Flg. 3 is a view similar to Fig. 2 showing a modified embodiment of the invention; and

Fig. 4 is a fragmentary plan view of a rotor member on an enlarged scale.

The particular motor shown in Figures 1 and 2 for the purpose of illustrating the invention is similar to the multi-pole synchronous motor disclosed in my copending application Serial No. 330,071, filed April 17, 1940, and assigned to the Westinghouse Electric 8: Manufacturing Company, although the invention may also be embodied in other types of small permanent magnet synchronous motors. This motor has a lamteeth Ii is the same on all of the poles, and the width of each tooth is preferably equal to half the tooth pitch, or in other words, the spaces between the teeth are equal in width to the teeth. The teeth II on each pole portion are uniformly spaced, but the teeth on successive pole portions are displaced from the positions corresponding turns to the magnet through the rotor without having to pass through the frame of the motor.

A 'still further object of the invention is to provide a multi-pole synchronous motor of the type described above in which the rotor is divided into two axially spaced sections which are resiliently Figure 1 is a somewhat diagrammaticplan view of a multi-pole synchronousmotor;

Fig. 2 is a vertical sectional view, showing the rotor and stator members of the motor;

to those of the preceding pole by a distance equal to one-half the width of a tooth, as clearly shown in the drawing.

Each pole portion has an exciting coil i2 wound on it andthe coils II are connected together to form two separate windings. Thus the coils of the pole portions 3, 5, 1 and 9 are connected in series to form one winding, the coils of the pole portions 3 and I being connected in opposition to the coils of the pole portions 5 and 9, while the coils l2 of the pole portions 4, 6, 8 and III are connected in series to form a second winding, with the coils of the pole portions I and 8 connected in opposition to those of the pole portions 8 and III. In operation, the two stator windings are excited with alternating currents which are approxlmately out of phase. This may conveniently be done, as shown in Fig. 1, by connecting the two windings in parallel to a singlephase'supply line II, with a capacitor llconnected in series with one of the windings. I

vTherotor-members It comprises a laminated core mounted on a hollow supporting spider it which is 'securedto the shaft ll. The shaft l1 maybe supported in suitable bearings in a frame of any. suitable construction. The rotor I5 is provided with a plurality of teeth I 8 uniformly as that of the stator teeth previously described. A stationary permanent magnet I9 is mounted within the hollow spider i6, so that its magnetic field extends through the rotor and stator members. If desired, any other suitable means of providing a substantially constant, unidirectional magnetic field might be utilized, such as a winding supplied with direct current.

The operation of this motor may best be understood by assuming the design to be such that the air gap flux produced by the permanent magnet IB is approximately equal to the maximum value of the flux produced by the exciting coils l2, which are supplied with alternating current. At the instant when the exciting current in the coils of the pole portions 3, 5, "I and 9 is passing through its maximum value, the flux produced by these coils is equal to the air gap flux produced by the permanent magnet I9. If we assume that the flux of the coils on the pole portions 3 and I is in the same direction as the permanent magnet fiux, the air gap flux under these pole portions will be approximately equal to twice the flux of the permanent magnet I9. At the same instant, the flux of the coils on the pole pieces 5 and 9 will be opposite in direction and of equal magnitude, so that the resultant iiux under these pole portions will be substantially zero. Under these conditions, the rotorwill tend to move to a position of minimum magnetic reluctance for the magnetic path through the pole portions 3 and 1, and will take up the position shown in Fig. 1, with the teeth l8 coinciding in position with the teeth of the pole portions 3 and 1. At this same instant, the current in the exciting coils of the pole portions 4, 6, 8 and ill will be passing through zero, since it is 90 out of phase with the current in the coils of the other pole portions, and the only flux under these poles, therefore, will be that of the permanent magnet, which is radially directed and of equal magnitude under all of these pole portions, so that it has no elfect on the motion of the rotor member.

A quarter of a cycle later, the current in the coils of the even numbered pole portions will be passing through its maximum value, while the current in the coils of the odd numbered pole portions will be passing through zero. Thus, the position of maximum flux will have shifted to either the poles l and 8 or the poles 6 and HI, depending on the direction of phase rotation, and the rotor member will accordingly move sufilciently to bring its teeth into the position of minimum reluctance for this new path, which, as will be clear from Fig. 1, will require a movement of the rotor of half a tooth width because of the relative displacement of the teeth of successive pole portions. In this way, the position of maximum magnetic flux will move around the motor and the rotor member will follow it, moving half a tooth width for each quarter cycle of the alternating current supply, so that the motor will rotate at a slow speed determined by the frequency of the supply and the number of teeth on the rotor.

In motors of this general type, as previously built and as shown, for example, in the motor disclosed in the above-mentioned copending application, the magnetic path for the flux of the permanent magnet has extended through the rotor and stator members and then through the frame of the machine back to the magnet. This construction results in a relatively long path, which necessarily has high reluctance and involves undesirably large magnetic losses. In addition, with this construction, the permanent magnet flux links the stator windings, and thus a damping effect is produced which tends to diminlsh the field strength of these coils, which is obviously undesirable. In order to avoid these difllculties, the present invention provides a con struction for the rotor member which provides a very much shortened magnetic circuit for the flux of the permanent magnet. Thus, as shown in Fig. 2, the laminated core of the rotor |5 is divided into two sections 20 and 2|, which are spaced apart axially of the motor and are separated by an annular spacer 22 of non-magnetic material, such as aluminum or brass. The magnet IQ is shaped so that its poles'lie opposite the separated sections 20 and 2| oi the rotor member, respectively, and the path of the magnetic flux from the magnet |9 then extends across a short air gap, through the rotor core section 20, across the air gap between the rotor and stator, through the stator core I, and to some extent through the frame 2, and then back across the air gap and through the other rotor core section 2| back to the magnet, as shown by the arrows in Fig. 2. It will be seen that a very short magnetic path is provided which has relatively low reluctance, since the two air gaps can be made quite short and the total length of the magnetic circuit is very much less than that provided in previous constructions, so that the magnetic losses are greatly reduced and a greater air gap flux can be obtained from a given permanent magnet. The flux which passes through the rotor and stator in this way does not link the exciting coils I2 of the stator, 50 that there is no damping effect on the flux produced by them, which improves the operation of the motor.

. It will be observed that the flux passes through the two sections 2|! and 2| of the rotor core in opposite directions, and for this reason it is necessary to displace the teeth on the two sections a distance equal to the width of a tooth, as shown in Fig. 4, so that the teeth of one section of the rotor are opposite the spaces between the teeth of the other section. By this arrangement, the relation of the rotor teeth of both sections to the fluxes in the air'gap and to the stator teeth is made such that the motor operates in the manner described above.

Since the flux of the permanent magnet passes twice across each of the two air gaps, i. e., the air gap between the rotor and stator cores and the air gap between the permanent magnet I9 and the spider IE, it is desirable to reduce the length of these air gaps as much as possible in order to keep the reluctance of the magnetic circuit low. By using the construction shown in Fig. 3, one of these air gaps can, in eifect, be eliminated, thus materially decreasing the reluctance. In this construction, the two halves 20 and 2| of the rotor core are mounted on supporting sleeves 23 and 24, respectively, and the two sections of the rotor are connected together by a spacer 25 of resilient non-magnetic material, such as rubber. The upper sleeve 23 is resiliently mounted on a hub 26 on the shaft H. by means of a ring 21 of rubber or other resilient material. The construction of the two sections 20 and 2| of the rotor and of the stator and the operation of the motor are the same as those described above.

In this embodiment of the invention, it will be apparent that when the motor is in operation, the two sections 20 and 2| of the rotor can move relative to the shaft independently of each other,

and they be drawn into contact with the stator core by the magnetic force at the points oi maximum fiux, as shown in Fig. 8. thus substantially eliminating one of the air gaps; As the the magnet It substantially constant and relatively low. I

It should now be apparent that a multi-pole, self-starting synchronous motor using a permanent magnet has been provided which is oi relatively simple construction, and which is a ma-- terial improvement'over .previous similar types or motors in that the magnetie'path tor the permanent magnet do: has been materially shortened in length. thus reducing magnetic losses and lowering the reluctance a the path. This results ina considerable M- in the periormance oi the motor. It should be-understood that the invention is not'ratricted to the particular construction or type of motor described for the W 0! illustration, applicablegenerallytoanytypeoi'ennllmotor using a permanent magnet or equivalent as a source of unidirectional magnetic fiux.

1 Itistobe,thereiore.thattheinvention is not limited to the exact details oi construction shown, but in its broadest aspects it includes all equivalent and embodiments which come within the scope oi the appended claims.

I claim as my invention:

1. An electric motor havim a stator member with a plurality oi salient poleportlons, each oi said pole portions having teeth on its peripheral surface and the teeth oi suceemive pole portions being relatively displaced circumierentially onehalt thewidth ot a tooth. a rotor member having ,uniiormly spaced teeth around its entire circumierence, the pitch of said rotor teeth being the same as that of the stator teeth. said rotor member being divided into two axially spaced sections separated by a non-magnetic spacer, said stator member extending axially past both sections of the rotor member, means'i'or producing a sub'-' stantially constant, unidirectional magnetic field passing radially through the rotor and stator I teeth, and means for producing alternating magsame as that of the stator teeth, said rotor mem-- her being divided into two axially spaced sections separated by a non-magnetic spacer, said stator member extending axially past both sections oi the rotor member, means for producing a substantially constant, unidirectional magnetic field passing radially through the rotor and stator teeth, said magnetic field being in opposite directions in the two sections of the rotor member, and means for producing alternating magnetic fields in the stator pole portions, thelmagnetic iields in adjacent pole portions being substantially 90 out oi phase.

3. An electric motor having a statormember with.a plurality or about pole portions, each of said pole portions haying teeth on its peripheral surface and the teeth of successive pole portions being relatively displaced circumi'erentially one-hall the width or a tooth, a rotor member having uniiormly spaced teeth around its entire circumierence, the pitch of said rotor teeth being the same as that oi the stator teeth, said rotor.

member being divided into two axially spaced teeth or the two actions of the rotor member being circumierentially displaced a distance equal to the width oi a tooth, said stator member extendingaxiallypastbothsectionsottherotor member, means for producing a substantially unidirectional magnetic field paw constant. radialiythroughtherotorandstator teeth,said magnetic field being in opposite directions in the twosecflonsoi'therotormembenandmeansi'or producing alternating magnetic fields in the stator pole portions. the magnetic fields in adiw 'n 30k portions being substantially 90' out o 4. An electric motor having a stator member with a pluralityot salient pole portions, each of said pole portions having teeth on its peripheral surface and the teeth of successive pole portions being relatively displac d circumierentiallyone-hall the width 0! a tooth, a rotor member having uniformly spaced teeth around its entire circmni'erence, the pitch of said rotor teeth being the same as that of the stator teeth, said rotor member being divided into two axially spaced sections separated by a non-magnetic spacer, the teeth 01' the two sections of the rotor member being circumierentially displaced a distance equal to the width oi a tooth, said stator member extending axially past both sections or the rotor member, a permanent magnet mounted centrally or the rotor member to provide a substantially constant, unidirectional magnetic field passing radially throughthe stator and rotor teeth, said magnet being disposed so that its magnetic field is in opposite directions in the two sections of the rotor member, and means for producing alternating magnetic fields in the stator pole portions, the magnetic fields in adjacent pole portions being substantially 90 out or phase.

5. An electric motor having a stator member with a plurality oi salient pole portions. each or said pole portions having teeth on its peripheral surface and the teeth 01' successive pole portions being relativelydisplaced circumierentially one-- half the width or a tooth, a rotor member having uniformly spaced teeth around its entire circumierence, the pitch, of said rotor teeth being the same as that of the stator teeth, said rotor member being divided into two axially spaced sections,

' said sections being separated by a spacer oi resilient, non-magnetic material and being mounted to permit them to move radially with respect to each other, said stator member extending axially past both sections or th rotor member,

means for producing a substantially constant, unidirectional magnetic field passing radially through the rotor and stator teeth, said magnetic field being in opposite directions in the two sections oLthe rotor member, and means for producing alternating magnetic fields in the stator pole portions, the magnetic fields in adjacent pole portions being substantially out or phase.

6. An electric motor having a stator member with a plurality of salient pole portions, ach of i said Me P rtions having teeth on its peripheral teeth of successive pole portions sections separated by a non-magnetic spacer, the

being relatively displaced circumferentially onehalf the width of a tooth, a rotor member having uniformly spaced teeth around its entire circumierence, the pitch of said rotor teeth being the same as that of the stator teeth, said rotor member comprising two axially spaced sections resiliently mounted on a shaft and separated by a spacer of resilient non-magnetic material to permit said two sections to move independently relative to the shaft, th teeth of the two sections of the rotor member being circumferentially displaced a distance equal to the width of a tooth, said stator member extending axially past both sections of the rotor member, a permanent magnet mounted centrally 01' the rotor member to provide a substantially constant, unidirectional magnetic field passing radially through the stator and rotor teeth, said m net being disposed so that its magnetic field is in opposite directions in the two section or the rotor member, and means for producing alternating magnetic fields in the stator pole portions, the magnetic fields in adjacent pole portions being substantially 90' out of phase.

EBERHARD MICHELSEN. 

